The invention generally relates to a steam turbine having a flow passage in which a moving blade is arranged. More preferably, it relates to one including a measuring system for measuring a vibration of the moving blade. The invention also generally relates to a method of measuring the vibration of a moving blade in a flow passage of a steam turbine.
The technical field of the invention is the measurement of blade vibrations in fluid-flow machines. Moving blades in fluid-flow machines are subjected to high loads. A vibration may be induced in them on account of alternating stresses, this vibration, if it lies in the vicinity of a natural vibration in the respective blade, leading to especially high mechanical stresses of this blade. In order to detect such especially high loads in good time, the vibration states of the blades are measured in different ways.
U.S. Pat. No. 4,996,880 discloses a steam turbine and a method of measuring the vibration of a moving blade in the flow passage of a steam turbine. Here, the vibration of the moving blade is measured by an acoustic signal. A Doppler displacement which is caused by the movement of the moving blade is measured with an acoustic sensor. Depending on the vibration state of the moving blade, a characteristic Doppler signal is obtained, so that, in inverse relationship to the Doppler signal, the vibration state of the moving blade can be deduced.
U.S. Pat. No. 4,518,917 shows a method of measuring the vibration state of moving blades, in which method the distance of the blades from the surrounding casing is measured. An impedance dependence of sensors arranged in the casing is utilized in this case. Depending on the distance of a moving blade from the sensor, an impedance change is obtained.
A further method of measuring the vibration of a moving blade has been disclosed by U.S. Pat. No. 4,934,192. Here, an axial deflection on account of an axial vibration of the moving blade is measured by two sensors being arranged symmetrically over a prominence on the tip of the moving blade in the rest state of the latter. The sensors are each designed as an electrical winding in which a voltage is induced depending on the distance from the prominence on the tip of the moving blade. In the event of an asymmetrical arrangement of the sensors relative to the tip of the moving blade, this asymmetrical arrangement occurring on account of an axial vibration of the moving blade, a differential signal is produced which characterizes the blade vibration.
In the paper xe2x80x9cA Review of Analysis Techniques for Blade Tip Timing Measurementsxe2x80x9d, S. Heath, M. Imregum, ASME publication 97/GT/218, presented at the International Gas Turbine and Aeroengine Congress and Exhibition, Orlando, Fla., USA, May 2, 1997, a method of measuring the blade vibration by means of a laser is described. However, this method relates solely to gas turbines.
The methods for the non-contact measurement of the vibration of a moving blade are either comparatively inaccurate or require a magnetizable material for the moving blade or the awkward and unreliable placement of a magnetic marking on the moving blade.
Accordingly, an object of an embodiment of the invention is to specify a steam turbine in which the measurement of the vibration of a moving blade is possible in a non-contact and reliable manner largely independent of properties of the moving blade. A further object of an embodiment of the invention is to specify a corresponding method of measuring the vibration of a moving blade.
According to an embodiment of the invention, an object which relates to a steam turbine may be achieved by a steam turbine having a flow passage in which a moving blade is arranged, and having a measuring system for measuring a vibration of a moving blade. The measuring system may include a transmitter for emitting a light beam to the moving blade and a receiver for receiving the light beam reflected from the moving blade, and the transmitter being separate from the receiver.
The use of an optical measuring system for measuring the vibration state of a moving blade in a flow passage of a steam turbine has hitherto not even been taken into consideration. This is due to the fact that, according to the prevailing opinion, the steam flowing in the flow passage makes optical detection of the moving blade virtually impossible on account of high scatter in the steam. In this case, however, measuring systems have hitherto always been based on a transmitter and receiver combined in a unit, so that in principle measurements are taken in backscatter. Such systems are used, for example, for measuring blade vibrations of gas turbines. They offer the advantage that only the casing part defining the flow passage need be interfered with for fitting the transmitter and receiver.
According to the findings of an embodiment of the invention, such an optical measuring system may now also be used in a steam turbine for measuring a blade vibration if the rigid concept of the coupled transmitter/receiver unit is dispensed with. This is because, in a suitable arrangement of transmitter and receiver, the proportion of scattered radiation which is caused by the steam can now be kept so low that a reliable measurement of blade vibration is made possible.
The transmitter and receiver need not be designed as a completely light-producing or light-converting unit; they may also be designed, for example, as a glass fiber cable and direct a light beam from a light source or to a converting unit, such as a photocell for instance.
The transmitter is preferably designed for emitting a laser beam. On account of its monochromasy and low divergence, a laser beam is especially suitable for the measurement.
The transmitter and receiver are preferably arranged in such a way that the transmitted light beam and the reflected light beam enclose an angle of reflection of at least 45xc2x0 with one another. The angle of reflection is also preferably greater than 90xc2x0. In such a large-angled arrangement, a good ratio of light beam reflected directly into the receiver to scattered radiation caused in the steam is obtained, since the proportion of scattered radiation drops with a larger angle.
The transmitter is preferably set in such a way that the transmitted light beam illuminates an area of less than 1 mm2 on the moving blade. High focusing or a low divergence of the light beam likewise encourages a low proportion of scattered radiation.
The receiver, in a casing defining the flow passage, is preferably arranged so as to be set back from the flow passage in such a way that, apart from the directly reflected light beam, at most a small proportion of scattered radiation reaches the receiver. By the receiver being set back in the casing, a diaphragm, as it were, is constructed, and this diaphragm essentially allows only such light to reach the receiver which spreads in a virtually rectilinear direction from the illuminated area on the moving blade to the receiver. As a result, scattered radiation spreading at other angles is mostly screened off before entering the receiver.
The moving blade is preferably made of a non-magnetic material. The moving blade is also preferably made of a titanium-based alloy. Here, the expression xe2x80x9cnon-magneticxe2x80x9d means that the material of the moving blade has no appreciable ferromagnetic properties. Especially in the case of such a material, a simple non-contact measurement by means of magnetic induction is ruled out. Such a material is, for example, a titanium-based alloy, which are used in new generations of steam turbines, in particular when the moving blades of the last stages of low-pressure parts are very large. However, such large blades especially are susceptible to vibration excitation and are loaded to a considerable extent by vibrations. Here, especially, a reliable monitoring system for measuring the blade vibration states must therefore be used. By means of the optical monitoring of the blade vibrations, this is also possible for such blades in a simple and reliable manner.
According to an embodiment of the invention, an object which relates to a method may be achieved by a method of measuring the vibration of a moving blade in a flow passage of a steam turbine, a light beam being directed onto the moving blade and being reflected from the latter at an angle of reflection greater than 45xc2x0 and directed to a receiver, and the blade vibration being calculated from the signal thus received.
The advantages of such a method follow in accordance with the above statements in relation to the advantages of the steam turbine.
The angle of reflection is preferably greater than 90xc2x0. The light beam used is preferably a laser beam.
The light beam is preferably directed onto a reflecting surface, lying on the moving blade, in such a way that the illuminated part of the reflecting surface is less than 50 mm2.
The moving blade is preferably made of a non-magnetic material, also preferably of a titanium-based alloy.